1. Field of the Invention
The present invention relates to systems for dispensing liquid chemicals and, more particularly, to automatic systems for switching from one source container of liquid chemicals to another source container.
2. Description of Prior Art
In the manufacture of integrated microelectronic components, commonly referred to as semiconductor chips, it is imperative to provide carefully controlled processing conditions to maximize the yield of acceptable products. In typical fabrication processes for microelectronic components, several hundred or more individual semiconductor chips may undergo the same manufacturing process at the same time and, accordingly, any processing upset or error can render useless a large number of the products. This is especially true in photolithographic processes where silicon substrates are chemically etched to form the topoqraphical patterns which are essential to operation of the microelectronic components.
In conjunction with chemical etching of semiconductor substrates, it is well-known to coat certain portions of the silicon substrate with a polymeric liquid chemical known as photoresist. Upon exposure to ultraviolet radiation, the photoresist forms a protective layer on the selectively coated portion of the substrate to protect that portion from chemical attack during the subsequent etching process. Such coating of photoresist must be thin but continuous over the area to be protected, otherwise a portion of the coated area will undergo etching and the electronic component will be rendered valueless. One cause of discontinuities in photoresist layers has been traced to air bubbles which are introduced to the process as the photoresist is dispensed. Accordingly, it is important in the fabrication of semiconductor microelectronic components to dispense photoresist in a manner which minimizes the amount of air entrained in the photoresist liquid.
In a typical microelectronic fabrication operation, photoresist is dispensed by pumping from relatively small bottles. This practice relates to the expense, toxicity and "shelf life" of the chemical, as well as to the necessity of maintaining purity of the product and the fact that the amounts of photoresist liquid which are periodically dispensed must be precisely controlled. Because the individual containers for the photoresist liquid are relatively small, it is periodically necessary to replace the containers in the dispensing equipment as they are emptied. Heretofore, the replacement operations have often permitted air to become entrained in the dispensing equipment, especially the pumps. For example, if a source container "runs dry" before it is replaced, air rather than liquid chemical will be drawn into the dispensing system and, subsequently, the air can cause imperfections in the coatings of photoresist upon the semiconductor substrates.